Abstract

Resistance toward antibiotics has become a problem on a global scale. Methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VRE) are a major cause of morbidity and mortality in hospitalized patients. To overcome resistance, many antimicrobial agents have been investigated and Traditional Chinese Medicinal (TCM) plants were also examined as source of alternative agents. Eucalyptus globulus Labill (Myrtaceae) was the most active plant among the 84 TCM plants tested against multidrug-resistant bacteria. The results indicate that non-volatile compounds contributed to the antimicrobial activity, instead of the essential oil. However, the essential oil of fruits of E. globulus exerted a strong inhibition against MRSA and VRE, in comparison to the oils of the leaves of E. globulus, E. radiata and E. citriodora. The fruit oil contained mainly aromadendrene, 1,8-cineole and globulol, which was determined by GLC/MS. Aromadendrene might be responsible for the antimicrobial properties, whereas 1,8-cineole and globulol exhibited low activities. Furthermore, the checkerboard assay demonstrated that combinations of 1,8-cineole and aromadendrene reduce the MIC in most cases in an additive way, whereas the time-kill assay indicates a synergistic effect. Chemical investigation of the essential oil of Kadsura longipedunculata Finet et Gagnep (Schisandraceae) detected delta-cadinene, camphene and borneol as the major components. The oil of K. longipedunculata exerted good antimicrobial and trypanocidal activities. Additionally, the oil was confirmed to have antioxidant and cytotoxic activities. Lipoxygenase inhibition and prostaglandin E2 production inhibition accounted for anti-inflammatory activity of the oil. Two major components of the oil, camphene and borneol, might contribute partially to the biological activity. Furthermore, the combination of camphene and borneol act synergistically against the tested bacteria observed by the checkerboard and time-kill methods. Bioassay-guided fractionation of Siegesbeckia pubescens Makino (Asteraceae) substantiated five bioactive compounds identified as daucosterol, pubetallin, siegeskaurolic acid, ent-16beta,17-dihydroxy-kauran-19-oic acid and 16alpha-(–)-kauran-17,19-dioic acid. The isolated compounds showed some different degrees of antimicrobial activity. Additionally, the combination of pubetallin and ent-16beta,17-dihydroxy-kauran-19-oic acid enhanced the inhibitory effect against MRSA observed by the checkerboard method. Antimicrobial activity of some TCM plants has been confirmed and the results validated the use of plants as infection remedies. The essential oils or isolated compounds either given alone or in combination could be promising agents to combat multidrug-resistant pathogenic microorganisms.